Refractory mold composition



' Patented Aug. 8, 1950 REFRACTORY MOLD COMPOSITION Bonheur Massna Weston, Detroit, Mich.

No Drawing. Original application February 2,

1946, Serial No. 645,238.

Divided and this ap- V plication June 23, 1947, Serial No. 756,560

1 Claim.

This invention relates to refractories for molds and cores used in casting molten metal. It is the principal object of the invention to provide siliceous refractories such as silicon dioxide and silicon carbide with a renewable bonding coating which will replace the water plasticizable coatings such as clay, cereal bonding materials, and the like, now commonly used. The invention substantially eliminates burning in or fusing of the refractory to the metal casting and the formation of detrimental scale and oxides on the surface of the casting which are disadvantages that attend the use of the water plasticized materials for bonding sand in the construction of molds and which appear to be due chiefly to the presence of moisture in the mold at the time of pouring the molten metal against the mold. The heat supplied by the molten metal converts the moisture to steam which develops pressures that interfere with the run and lay of the metal, at times causes formation of cavities in the casting and provides an oxidizing atmosphere at the surface of the casting which favors formation of oxides on, and the fusing of sand to, the casting surface.

This application is a division of my eopending application Serial No. 645,238 which was filed Feb. 2, 1946 now Patent 2,444,413 issued July 6. 1948.

In the case of cores where the refractory and its bond is subject to more heat than in the case of molds and where moisture is driven off by baking before casting, the clays and other materials now used as binders form a debris that makes the used refractory unsatisfactory for re-use and diflicult to recondition. This disadvantage is avoided by my invention.

I have discovered that fine grained siliceous refractories such as silicon dioxide and silicon carbide may be provided with a relatively longlived and replenishable coating which will provide the green strength required in molds and cores without the use of water, decompose into a substantially reducing atmosphere at the surface of the mold during the pouring operation' without destructive gas or vapor pressures, supply the required bond strength throughout the casting operation and readily break up at the completion of the operation, and leave the residue of coating adhering to the refractory grains in condition for re-use and susceptible to replenishment. By means of my invention, substantially all of the refractory is preserved in condition for re-use for both cores and molds, and used core sands need not be discarded as in the present practice. Furthermore, castings made with molds utili in my invention have surfaces exceptionally clean and free from scale andfused refractory. These beneficial results flowing from the use of my invention very substantially reduce the cost of castings by eliminatin: frequent replacement of refractory and the attendant substantial cost of handling. The machining and cleaning operations necessary to free castings of scale and fused refractory which occur in present practice, and scrap castings which are due in the present practice to imperfections brought about by scale, fused refractory and cavities, are also very substantially reduced.

My invention provides each of the refractory particles or grains with a thin coating of plasticizable nonthermosetting resin. By the term nonthermosetting it is meant that the resin must not be converted physically when subjected to temperatures below its combustion temperature. In this instance the resin should be combustible at a temperature within the range 600 F. to 800 F. At temperatures above the combustion temperature and in the presence of conditions which support combustion, the resin may be consumed substantially completely. Thus, at the surface of the mold or core the resin may be entirely consumed, but resin farthur back in the mold or core which is not subjected to temperatures above the combustion temperature or exposed to conditions which permit combustion should be unchanged or unconverted" in order to assure an easy shake-out" and to permit re-use of the coated sand particles. This is done by dissolving or suspending the resin in a combustible nonaqueous solvent which will not chemically react to change the chemical constitution of the resin. A quantity of the resin solution composed of approximately equal parts by weight of resin and solvent, and approximately 2 to 4% by weight of the refractory to be treated, is added to the refractory and thoroughly mixed therewith until each grain of the refractory is coated with a film of the resin solution. When this is accomplished the mass is aerated or dried while being'mixed or agitated, at a temperature suflicient to evaporate substantially all of the solvent. This will leave a thin, relatively dry film of the resin firmly adhered to the surfaces of the refractory grains. After drying, the coated refractory grains are not sticky or tacky and will flow with substantially the same freedom as the bare refractory. At the time of use for constructing molds, solvent or plasticizer is applied to the coated refractory for the purpose of plasticizing or softening the surface only of the coating on the refractory sufficiently to develop the cohesiveness between particlesthat is required to furnishthe green bond strength needed for the mold. After addition of the plasticizing solvent, the coated refractory is formed into molds or green cores in the usual manner which are then ready for the casting operation. 'If it is desired to provide the mold or core with greater strength than the customary green bond strength. and thisis usually desirable in the case of cores, the mold or core may be baked after forming to develop such dry bond strength as may be required, and such strength may be increased by including some thermosetting resin or drying compound in the plasticizing solvent. Where the core is baked before using, the baking operation will drive off the plasticizing solvent and thereby strengthen the bond provided by the resin. In the case of molds which ordinarily need not be baked before using, most of the plasticizing solvent will be present in the mold at the time of the pouring operation and the molten metal will cause early ignition of the plasticizing solvent and thus prevent development of detrimental pressures. The heat of the molten metal will also, by eliminating the solvent as stated, cause the strength of the bond furnished by the refractory coating to increase sufliciently to enable the mold to properly contain the casting. Sufficient of the resin coating will be consumed during the casting operation by chemical reaction with the oxygen present to form a reducing atmosphere. The nonthermosetting character of the resin permits shrinkage of the casting and easy break up of the mold.

After the casting operation, the refractory will be found to have retained a very considerable portion of its resin coating and to be substantially free of useless and undesirable debris. The resin which remains upon the refractory is unchanged chemically and retains its plasticizable quality. This refractory may be easily reconditioned by applying more resin coating to it to replace that which has been lost in the casting operation. Experience demonstrates that an amount of resin equal to .2 to .5% of the weight of the sand will be. lost in the casting operation and this may be replenished by mixing with the used refractory more resin solution having the proportion of parts of resin to 60 parts of solvent. The replenishing solution should be in the proportion, on the average, of .5 to 1% by weight of the weight of the refractory to be treated, and always sufficient to replace the actual loss. Thus, the refractory of this invention may be defined as silicon dioxide or silicon carbide particles, each particle having a resinous surface film which is combustible in the range of 600 F. to 800 F. approximately and is plasticizable and nonthermosetting at temperatures below that temperature at which it is decomposed by combustion and subsequently replasticizable after having been subjected to such temperatures.

I have discovered that in order to fix the original resin coating upon the refractory grains prior to use of the coated refractory for a mold, it is necessary that the solvent be driven from the resin of the initial coating, otherwise there will not be sufiicient adherence between the resin and the refractory particles. The drying operation has the effect of firmly fixing the resin film on the refractory particles. Then at the time of preparing the mold, solvent or plasticizer is added to plasticize only the outer surface of the fixed resin coating and leave undisturbed the adhesive bond between the surface of the refractory particle and the resin coating. The plasticizing of the outer surface of the resin coating upon the refractory particles develops cohesiveness between the surfaces of the coatings on adjacent particles without disturbing the adhesion of the respective coatings to their respective refractory particles. As the casting operation does not result in the destruction or consumption of all of the resin coating upon the refractory particles, but leaves a substantial residue thereon, it is not ordinarily necessary, in replenishing the coating of refractory that is to 4 be promptly re-used, to again dry the coated refractory after the replenishing solution has been applied because the replenishing coatlne will readily cohere without drying to the resin coating which remains upon the refractory, and will become fixed on uncoated grains by the heat developed in subsequent casting operations.

However in case the used refractory contains more than about 20% uncoated grains. either because of consumption of resin in the casting operation or addition of uncoated refractory, it will usually be found desirable to dry after replenishment and before use in order to provide a suificiently high percentage of grains containing a fixed resin coating to obtain the necessary bond strength.

The bond strength of the mold or core may be increased by adding to the standard or usual grain size refractory some finely comminuted refractory such as silica flour. This has the effect of increasing the total surface area of refractory in a given volume that is capable of supporting the bond coating of resin. The amount of fine material to be added can be gauged, as in present practice where clay and cereal binders are used, by standard tests for gas permeability applied to sample or test molds; Care should be taken to avoid use of fine refractories which contain any substantial amount of alkaline substances or which are of highly absorbent nature if the refractory is to be used for :ores.

The resins used for providing the coatings described should be combustible in the range 600 F. to 800 F. approximately. They should be nonthermosetting and soluble or dispersible in nonaqueous inflammable solvents, which may be driven off or evaporated readily at temperatures below 300 F., have an ignition point preferably above F., and which do not chemically react with the resin or change its nonthermosetting character. It is preferable to use resins which have a relatively high carbon and low hydrogen content in order to take up the oxygen present and minimize the formation of water upon decomposition during combustion. It is also desirable but not essential to catalyze the resin with metallic oxides (not less than' or 1% of weight of resin) as such oxides appear to have the effect of inducing an earlier combustion of the carbon of the resin and producing nonoxidizing atmosphere at the surface of the casting. Oxides of silver, nickel, cerium, vanadium, manganese, chromium, copper, cobal and iron are suitable.

Aromatic hydrocarbon solvents for the resin are preferable and those having substantially the following properties will be suitable:

The following specific solvents may be used:

Turpentine Toluol Benzine Xylol Methyl methacrylate Phenol monomer Ethylene dichloride Coal tar naphtha Carbon disulphide Benzol The fixing of the inital film on the refractory may also be expedited and its adherence to the refractory surface increased by the inclusion of a small amount of the drying oil to the primary resin solution. Drying compounds, such as alkyd resins, tung oil, linseed, perilla, and soy bean oil may be used. In general, a resin solution having approximately 40 partsof resin to 60 parts of solvent by weight will serve. The following general formula will be found to be suitable:

Resin 40.0 by weight Solvent 57.0 by weight Drying compound 2.9 by weightoptional Plasticizer, such as,

Polybutene Polyvinyl chloride or Dlbutyl phthalate 0.1 by weight-optional The quantity of resin solution to be used should be from approximately 2 to approximately 4% of the weight of the refractory to be treated. Refractories of finer grain size, which will have a greater total surface area in a given volume, will require greater proportions of the resin solution for a satisfactory coating. than refractories of larger grain size and less total surface area per unit of volume. v

After the initial resin coating has been dried upon the refractory the coated refractory is prepared for the mold by mixing again with solvent or plasticizer for the purpose of plasticizing the surfaces only of the resin coatings on the refractory grains so that they will cohere sumciently to develop-the green bond required in the mold.

- The amount of plasticizer or solvent for this use should be approximately .75 to 1.5% of the weight, of the refractory to be treated .and the plasticizing action will be improved if the plasticizing solution contains approximately by weight of the coating resin.

The replenishing resin solution for treating the refractory after use in the casting operation and reconditioning it for further use should be in the proportion of approximately 2 parts of a resin of the type hereinabove described to 3 parts of solvent by weight, which is a formula that enables the automatic replacement of the average loss of resin by calculating and replacing the actual loss of solvent, andapproximately on the average of $5 of 1% by weight of the refractory to be reconditioned, care being taken to add sumcient to replace the actual loss.

Hydrocarbon resin produced in petroleum cracking is typical of the resins suitable for use in the instant process. A general specification of a typical resin is as follows:

TABIE I Chemical constituents Per cent Carbon 84.42 Hydrogen 8.53 Oxygen 1.86 Nitrogen 1.12 Sulphur 3.50 Moisture 4. or less llineral matter (substantially Hydrocarbon composition /100/5 -16.0 Ductility at 185 2 Susceptibility factor 42.6 Flash point 635 F. Fusion point (R 8; B) 300 to 350 F.

It is to be understood that the appended claim is intended to include variations in the specific I directions given occurring to those skilled in the art which do not forego the essentials of my invention. n

What is claimed is: f A bonding refractory for use in molds and cures for casting molten'metal consisting of siliceous l particles selected from the group consisting of sil- Number Name Date 553,688 Prinzler Jan. 28, 1896 1,285,081 Elliot Nov. 19, 1918 1,561,956 Thomas Nov. 17, 1925 1,778,329 Mason Oct. 14, 1930 2,029,288 Bray Feb. 4, 1936 2,047,297 Stahl July 14, 1936 2,086,968. Stafford July 13, '1937 2,131,205 Wells Sept. 27, 1938 2,201,321 I Robie May 21, 1940 2,274,618 Remy Feb. 24, 1942 2,328,622 Crawford --Sept. 7, 1943 3 2,409,437 Lacrosse Oct. 15, 1946 icon dioxide and. silicon carbide, the individual 1 particles being entirely coated witha hydrocarbon resin substantial y as defined in ,Table I.

BONHEUR MASSENA WESTON.

I I REFERENCES crrno The following references are of record in the file of this patent: I

UNITED STATES PATENTS OTHER REFERENCES Synthetic Resins and Allied Plastics" by Barry, Britton, Langton and Mon-ell. P8868 1,09, 309 and 310. 

